I bet you if GoW for the PC launched the same day as the 360 version, It would have sold 3-4x as many copys.
I'll bet you that Epic has more resources available to do the kind of research necessary to make that kind of prediction than you do. Somehow, this is not the conclusion that they came to. Though given that it sold 5 million on Xbox 360 alone, it seems pretty unlikely that a simultaneous PC release could have boosted it all the way to 15-20.
I have my own hypothetical claim - the PC port might have sold more copies, but a larger number of PC-360 users who in the real world bought the 360 version would have just pirated the PC version instead, meaning fewer sales overall. It's the only explanation I can think of for why the PC port took so long - PC ports of modern games are relatively straightforward, and it's not as though they had a huge optimization budget.